sysinfo: Allow displaying more info on startup

At present only the model name is shown on start. Some boards want to
display more information. Add some more options to allow display of the
manufacturer as well as the version and date of any prior-stage
firmware.

This is useful for coreboot, at least. If other boards have more
information to display, it is easy to add it, now.

Signed-off-by: Simon Glass <sjg@chromium.org>